1. Field of the Invention
This invention relates to a piston of aluminum alloy for internal combustion engines, and is particularly but not exclusively applicable to piston for direct-injection-type diesel engines.
2. Description of the Prior Art
A combustion chamber 2, for example, illustrated in FIG. 4, which is provided in the piston head 1a of a piston 1 for internal combustion engines, is locally overheated by hot gases under high combustion pressure, so that cracks are often initiated at the edge 2a of the combustion chamber 2, and propagated in the radius direction of the piston 1.
To overcome such problem, a preform 3 of ceramic fiber, which has a predetermined density, is annularly formed to correspond with the edge of the combustion chamber, and set in a metal mold. After a molten aluminum alloy is poured into the metal mold, the piston is casted under a predetermined pressure so that the molten alloy may be penetrated into the preform 3. Thus, the strength of the edge of the combustion chamber is much improved
However, in the pressure casting, the pressure of 600 to 1200 kg/cm.sup.2 is applied to the molten alloy, so that the used casting equipment and the metal mold must be endurable under such high pressure. Thus, the cost of the piston becomes extremely heavier than that of a piston obtained by means of gravity casting. Further, in the pressure casting, a segmented core can not be used as the melt penetrates into the separated part of the core. Therefore, it becomes impossible to make an undercut 4 on the inside of the piston.
Generally, it is preferable that each filament of the ceramic fiber used for such preform is arranged as much as possible in a direction perpendicular to the propagation of cracks when a metal is reinforced by the ceramic fiber, and thus, the filaments should be arranged along the circular edge of the combustion chamber. However, it is difficult to arrange the filaments of the ceramic fiber as much as possible in such direction when the preform 3 is annularly formed during the production of preform 3. Therefore, the preform 3 can not completely interfere with the cracks initiated and propagated in the direction perpendicular to the annular edge 2a of the combustion chamber 2.
In the meanwhile, such difficulty derives from the preform 3 being formed in such a manner that a slurry including the ceramic fiber of short filaments is filtered under reduced pressure. That is, the filaments passing through the filter are apt to be arranged in a direction for being filtered, and thus, the filaments are arranged rather at random on a surface which is parallel to the filter, so that it is difficult to arrange the filaments annularly.
If the short filaments are substituted by long ones, it seems easy to arrange the filaments annularly, but it takes plenty of time to arrange the filaments, and also, it becomes difficult to adjust the volumetric ratio of the filaments, so that the cost of the preform will rise.